Fear is an important survival mechanism and so too is the ability to inhibit fear when it’s no longer needed. In order to counter-balance fear, the brain engages in fear extinction.
In this process, memories are formed during non-fearful experiences with similar environmental elements. These non-fearful memories then compete with the original fear memory.
Now, in a new paper published in the journal Nature Neuroscience, the University of Queensland’s Professor Tim Bredy and his colleagues show that the ability to extinguish fearful memories in this way relies on the flexibility of your DNA.
“DNA can adopt a variety of different structures,” says Dr Paul Marshall, a researcher at UQ’s Queensland Brain Institute and lead author of the study.
“The most common and most widely recognized form is the ‘B-DNA’ double helix, which twists in a clockwise direction.
But, with a slight rearrangement of how DNA base-pairs connect with one another, DNA can form other helical structures, such as Z-DNA.”
Z-DNA is a counter-clockwise twisted version of B-DNA, he explains. Imagine for a moment, that each of your hands is a DNA strand, the thumbs the bases. If you hold both hands out in front of you, palms out, so that your thumbs touch, this is how two bases connect in B-DNA.
If you now flip your wrists so that your palms face inward and your pinkies touch, this is how bases flip-out during Z-DNA formation.
If you keep rotating your hand and now re-join the thumbs this is what happens when Z-DNA is stabilized into a new twist.
Z-DNA occurs over short regions and only certain sequences can turn inside-out like this. For a long time, no one knew why it existed at all.
“We now know that Z-DNA appears wherever genes are being turned on,” says Dr Marshall. “It’s a marker of gene activity.”
“Scientists have also noticed a connection between Z-DNA and certain diseases, including cancer, and high levels of Z-DNA have been found in the brains of people who had Alzheimer’s Disease.”
This potential link with memory intrigued Dr Marshall and Professor Bredy, especially since the formation of fear extinction memories involves rapid changes in gene activity.
To find out more, they turned their attention to an enzyme called ADAR1, which recognizes and latches onto Z-DNA. ADAR1 is known to play a role in RNA editing, which is important for modifying protein functions in the cell.
Evidence also suggests that ADAR1 can convert Z-DNA back into B-DNA.
“ADAR1 is doing a lot of things at once, but that’s what makes it interesting,” says Dr Marshall.
He and his colleagues turned off the ADAR1 gene in mice, specifically in a part of the brain known to play a role in fear extinction. As a result, although the mice could still form fear memories, they were unable to form non-fearful memories.
In short, they lost the capacity for fear extinction. The researchers observed a similar effect when they mutated ADAR1, so that it didn’t work very well.
The findings suggest that Z-DNA forms during fear then, during fear extinction, ADAR1 binds to that Z-DNA and carries out two important jobs: it rapidly increases RNA editing and then flips Z-DNA back into B-DNA.
“It seems that the more easily you can switch between DNA structures, the more plastic your memory is,” says Dr Marshall.
“Flexibility of DNA structure, flexibility of memory.”
This enables an agile response to our environment, he adds. “Fear memories need to be plastic. They can be very useful for survival, but they can also get in the way of normal functioning.”
The balance between fear and fear-extinction is critical to cognitive flexibility, says Professor Bredy. Indeed, the impairment of fear extinction is a key feature of PTSD and phobias. The more we understand about how fear extinction works, the more chance we have of finding better treatments for those conditions.
Memory consolidation is the time and protein synthesis-dependent stabilization process that takes place after learning to convert short-term memory into long-term memory.
Consolidated memories are stable but mutable and can return to a labile state when activated during retrieval, requiring a restabilization phase known as reconsolidation to endure. Consolidation and reconsolidation share several molecular properties and result in persistent synaptic changes for memory storage. However, they are distinguishable processes that serve different biological purposes.
Brain-derived neurotrophic factor (BDNF) regulates neurogenesis, neuronal differentiation, maturation and survival during development (Park and Poo, 2013).
BDNF is enriched in the adult’s brain hippocampus and cerebral cortex (Conner et al., 1997), where it exerts neuroprotective effect, enhances synaptogenesis and neurotransmission, and mediates activity-dependent synaptic plasticity (Poo, 2001; Panja and Bramham, 2014).
BDNF is one of the most commonly studied proteins in memory research. In fact, in the last two decades plenty of studies described the participation of this neurotrophin in the acquisition, consolidation and long-lasting storage of different memory types (Ou et al., 2010; Martínez-Moreno et al., 2011; Bekinschtein et al., 2014).
In particular, the role of BDNF in memory extinction, a process induced by repeated non-reinforced reactivation resulting in an inhibitory memory that opposes the original learned response, is well documented (Peters et al., 2010; Andero and Ressler, 2012; Xin et al., 2014).
Maybe because of that, BDNF involvement in memory reconsolidation has been less investigated, although recent studies indicate that BDNF does play an essential function in this process, too.
BDNF and Memory Reconsolidation
Neurotrophins are key regulators of long-term synaptic modifications. They are synthetized and secreted in an activity-dependent manner, acting locally at active synapses to enhance neurotransmission efficacy (Canossa et al., 1997; Poo, 2001).
In particular, BDNF synthesized at dendrites is critical for LTP, a form of long-term plasticity and a putative cellular mechanism for memory storage (Morris et al., 1986), mediating post-translational modifications at pre- and post-synaptic terminals and regulating local translation.
BDNF contributes also to structural changes in synaptic spines (Tanaka et al., 2008) and sustains LTP even when protein synthesis is inhibited (Pang et al., 2004). Reactivation of potentiated synapses can render LTP sensitive to protein synthesis inhibition once again, indicating that LTP stability is a function of neuronal activity level (Fonseca et al., 2006) and suggesting that molecular mechanisms involved in LTP might also be important for reconsolidation.
It is not surprising then that BDNF can also mediate reconsolidation-induced plasticity helping to remodel synapses activated by retrieval without affecting other circuits. In fact, memory reconsolidation depends on several molecules involved in LTP maintenance, such as Zif-268 and PKMζ (Lee et al., 2004; Rossato et al., 2019). In this respect, Samartgis et al. (2012) found that BDNF administration following a reminder session facilitates avoidance memory in chickens, showing for the first time that BDNF is indeed necessary for memory reconsolidation.
In agreement with these results, reactivation-induced fear conditioning memory enhancement requires hippocampal BDNF expression in stressed rats (Giachero et al., 2013). Also, post-retrieval intra-CA1 spermidine administration lengthens contextual fear memory duration through a mechanism that depends on hippocampal BDNF maturation as well as on the interaction between this neurotrophin and its main receptor, tropomyosin-related receptor kinase B (TrkB; Signor et al., 2017).
Further, increased BDNF mRNA and protein levels as well as TrkB activation in the insular cortex accompany hippocampus-independent conditioned taste aversion (CTA) memory retrieval, and interfering with BDNF synthesis in this cortex after reactivation causes amnesia. Notably, post-retrieval intra-insular cortex BDNF administration reverses CTA impairment and enhances weak CTA memory retention (Wang et al., 2012).
Reactivation of fear extinction memory also increases BDNF levels and TrkB phosphorylation in the rat hippocampus while intra-CA1 administration of function-blocking anti-BDNF antibodies after extinction memory retrieval hampers extinction memory reconsolidation causing reinstatement of the extinguished fear. Importantly, hippocampus BDNF signaling activation preserves the learned extinction response when extinction memory reconsolidation is blocked (Radiske et al., 2015).
This suggests that the mnemonic representation that controls behavior during retrieval is the one that gets weaken, as proposed by the trace dominance theory (Eisenberg et al., 2003), and also that BDNF signaling is sufficient to reconsolidate the prevailing memory.
In line with these results, BDNF Val66met polymorphism, which is associated with hippocampus plasticity and BDNF trafficking (Egan et al., 2003), impairs conditioned fear memory storage when a brief fear reactivation session is followed by extended extinction training in humans (Asthana et al., 2016).
The participation of BNDF in memory reconsolidation is not restricted to distressing memories. Recognition memory, a major component of declarative memories, provides the ability to identify previously encountered events, objects and individuals.
In rats, object recognition memory (ORM) maintenance requires de novo hippocampal protein synthesis after retrieval, but only when novelty is perceived during reactivation, suggesting that reconsolidation recruits the hippocampus to incorporate new information into the active recognition trace (Rossato et al., 2007). In neurons, BDNF is synthetized as a precursor peptide, proBDNF, which is stored or further cleaved to produce mature BDNF (Pang et al., 2004; Hwang et al., 2005).
In the hippocampus, pro and mature forms of BDNF are abundant in presynaptic terminals of glutamatergic neurons and after release they act locally through their binding to p75 neurotrophin receptor (p75R) or TrkB, respectively. Activation of p75R by proBDNF facilitates LTD at CA1 synapses (Woo et al., 2005), and proBDNF extracellular conversion to BDNF by the tissue plasminogen activator (tPA)/plasmin system is essential for sustaining LTP (Pang et al., 2004).
In accordance with these observations, ORM reconsolidation modifies hippocampal synaptic efficacy in rats, inducing a rapid depotentiation phase that occurs around 1.5 h after retrieval and is followed by a synaptic potentiation stage taking place ∼4.5 h thereafter (Clarke et al., 2010). Consistent with these findings, ORM reconsolidation is also accompanied by post-retrieval proteolysis of proBDNF, which augments BDNF levels and promotes functional BDNF/TrkB interaction in the hippocampus to restabilize the reactivated representation and incorporate new declarative information concurrently (Radiske et al., 2017b). PKMζ is a constitutively active PKC isoform highly expressed in the hippocampus. that would be responsible for sustaining long-term memory storage (Sacktor, 2008). BDNF modulates PKMζ turnover (Kelly et al., 2007) and maintains PKMζ-dependent late-LTP in the hippocampus even in the absence of protein synthesis (Mei et al., 2011).
Interestingly, we recently demonstrated that BDNF mediates ORM reconsolidation-induced plasticity through PKMζ, which, in turn, regulates AMPAR trafficking at postsynaptic densities in the dorsal hippocampus to update the reactivated memory trace (Rossato et al., 2019). Figure 1 shows a model of the molecular mechanism that might be mediating ORM reconsolidation. The study of Rossato et al. (2019) also provides behavioral, pharmacological and electrophysiological evidence supporting the idea that disrupting the reconsolidation process causes memory erasure.
Importantly, blocking BDNF maturation as well as inhibition of BDNF downstream effectors after retrieval delete the reactivated recognition memory trace but leave dormant ORM intact, suggesting that memory destabilization specifically affects reactivated synapses and that BDNF modulates local synaptic remodeling to restabilize the updated trace. Notwithstanding this, other studies found that BDNF involvement in memory processing is restricted to memory consolidation and plays no role in reconsolidation (Lee et al., 2004; Barnes and Thomas, 2008; Lee and Hynds, 2013).
This discrepancy must be due to the fact that most of these studies employed pre-training or pre-reactivation infusions of BDNF antisense oligodeoxynucleotides to hinder BDNF expression by knocking down proBDNF mRNA translation but were unable to affect the conversion of already available proBDNF to mature BDNF, which is essential for memory reconsolidation (Radiske et al., 2015).
Implications of Bdnf Signaling Manipulation During Memory Reconsolidation
Determining the molecular basis of retrieval-induced cognitive processes is necessary not only to understand the dynamics of the memory storage process but also to prevent memory decline and treat disorders associated with the persistent reenactment of maladaptive recollections (Kida, 2019). The studies reviewed here suggest that targeting BDNF is a promising adjuvant to help patients recontextualize disturbing memories during reconsolidation-based therapies.
Memories about emotionally arousing events are usually persistently stored which, in some cases, lead to intrusive and distressing recollections that may result in anxiety, phobia or other types of disarrays such as post-traumatic stress disorder (PTSD; Ehlers, 2010). A potential tool to treat the exacerbated avoidance responses caused by the expression of fear memories at the core of some phobic behaviors is to disrupt its reconsolidation (Beckers and Kindt, 2017).
However, this intervention presents limitations because, as mentioned above, retrieval does not always induce memory destabilization. Extinction-based psychotherapies are an alternative strategy to reduce traumatic memory expression, but their effects are not persistent and the current challenge is to maintain the extinction memory over time (Vervliet et al., 2013). In this respect, it was recently proposed that enhancing reconsolidation of extinction memory could be a viable strategy to avoid its decay (Radiske et al., 2015; Rosas-Vidal et al., 2015).
Findings from the last decade show that BDNF modulates reconsolidation of both aversive and extinction memories (Wang et al., 2012; Giachero et al., 2013; Radiske et al., 2015; Signor et al., 2017). Overall, these studies suggest that drugs interfering with BDNF signaling during reconsolidation of aversive memories could help to impair its retention, while approaches that activate BDNF/TrkB pathways after extinction memory retrieval may promote its persistence, preventing reappearance of the fear response.
Antidepressants can influence BDNF levels bidirectionally. For example, a single dose of the serotonin re-uptake inhibitor fluoxetine decreases BDNF expression (Coppell et al., 2003) and attenuates aversive memory persistence in rats (Slipczuk et al., 2013), but chronic administration of the same agent upregulates BDNF mRNA levels (Coppell et al., 2003).
These observations suggest that the patient’s medication history can deeply influence the outcome of therapies based on memory reconsolidation. Acute interventions with antidepressants after traumatic memory reactivation may disrupt its reconsolidation, helping to reduce the disturbing symptoms.
On the other hand, reconsolidation interference should be avoided in patients that are being treated for depression with drugs able to augment BDNF function. In this case, extinction-based treatments could be more effective, since enhanced BDNF signaling promotes fear memory extinction (Peters et al., 2010) as well as its persistence through memory reconsolidation (Radiske et al., 2017a).
An alternative strategy that is being study to enhance PTSD exposure therapy efficacy consist of coupling extinction sessions with physical exercise, which increases peripheral BDNF levels (Powers et al., 2015).
In this respect, it has been recently shown that lactate mediates the facilitatory effect of physical exercise on cognition by upregulating hippocampal BDNF expression (El Hayek et al., 2019). Because the healthy human brain can uptake systemically administered lactate (van Hall et al., 2009), it would be interesting to evaluate lactate as a putative therapeutic molecule to reduce fear relapse by potentiating extinction through the enhancement of extinction memory reconsolidation.
Alzheimer’s disease (AD) progression has also been associated with impaired reconsolidation and reduced BDNF signaling (Hock et al., 2000; Ohno, 2009), suggesting that increasing BDNF function during reconsolidation could partially counteract declarative memory deficits in AD patients.
In this respect, transcranial direct current stimulation, which activates signaling downstream BDNF and elicits LTP-like mechanisms in rats, improves episodic and semantic memories in AD patients (Cocco et al., 2018) a result in line with earlier findings showing that the blood-brain barrier permeable TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) ameliorates cognitive decline in AD animal models (Devi and Ohno, 2012).
University of Queensland